From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 46DF4376BF4 for ; Tue, 13 Jan 2026 10:15:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768299334; cv=none; b=DW3k7E+ccBIrlSUFRlKiC0K5PDRUQA4Upi6EPvW4xrJ3ZscMUAXNEHVM24/DBzkNRfDeRIhuTkuRfilwut16HZGCEYC3gT/ylT7kMGfje0PsTv24HDfwdKscjXPffGBhiDkqydkWPiuQk5USp9o0UvbQ8UnC0FLhnZSgTPkLUso= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768299334; c=relaxed/simple; bh=deRUAWIkgjHzXVV3cTmMvD4gr7oy47IkI84pINNcAqQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=cvJQMNd7hywMbTsW5zH+Y/k8yXO7OM0cWLh/WMOKzgCyEp/e3f3K+0dcE0eKrAVugAdgxlzeUwRXzuE6NnAGG9B9fcqprj3lsz3QjJnUN71itQHLB0SxcX7SKU/ILi3IlGqdjxVIQtuQuZeDVJHvDO3KvHctShivy8rf5grKflc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JKchUiTq; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JKchUiTq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5468AC116C6; Tue, 13 Jan 2026 10:15:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768299334; bh=deRUAWIkgjHzXVV3cTmMvD4gr7oy47IkI84pINNcAqQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=JKchUiTqrBXmedCjGI0bAxn9puHSObsRiojxEBeoJJxbhHibVzSCf/lyzADGXCpoy n1RMGbqC6tGw97/p1n93+GR5kJpE3NTQL0qYksCYAT+ZIhKeCrm0zahbCGScUyxerz MqwJSCf7hocs3WwkqnwrrmqMej+7D4Y39Xsr581gkUSdzH5svy4pIlXoctvzEudqD2 hTQ1WLTwFbQMVmxxAG56nxOBnUq5/jWTiKaE3tUwqqGmDGh4YTNyVYz0hL8oKt6vyE 6WmhxjeMQPdzfVEmwd0FE+Hc29ax7w7YJSuKRQa9Ga2+mdi8NjeeFA9xCt9/+axWWf S2ssqlJLg1OXg== Message-ID: <7963534f-cce8-4330-8a67-3f31bd6b2166@kernel.org> Date: Tue, 13 Jan 2026 11:15:29 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/8] Introduce a huge-page pre-zeroing mechanism To: Li Zhe Cc: akpm@linux-foundation.org, ankur.a.arora@oracle.com, fvdl@google.com, joao.m.martins@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, mjguzik@gmail.com, muchun.song@linux.dev, osalvador@suse.de, raghavendra.kt@amd.com References: <20260113063736.29694-1-lizhe.67@bytedance.com> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/13/26 07:37, Li Zhe wrote: > On Mon, 12 Jan 2026 20:52:12 +0100, david@kernel.org wrote: > >>> As for concern (4), I believe it is orthogonal to this patchset, and >>> the cover letter already contains a performance comparison that >>> demonstrates the additional benefit. >>> >>>> I did see some comments in [1] about QEMU supporting user-mode >>>> parallel zero-page operations; I'm just not sure what the current >>>> state of that support looks like, or what the corresponding benchmark >>>> numbers are. >>> >>> As noted above, QEMU already employs a parallel page-touch mechanism, >>> yet the elapsed time remains noticeable. I am not deeply familiar with >>> QEMU; please correct me if I am mistaken. >> >> I implemented some part of the parallel preallocation support in QEMU. >> >> With QEMU, you can specify the number of threads and even specify the >> NUMA-placement of these threads. So you can pretty much fine-tune that >> for an environment. >> >> You still pre-zero all hugetlb pages at VM startup time, just in >> parallel though. So you pay some price at APP startup time. > > Hi David, > > Thank you for the comprehensive explanation. > > You are absolutely correct: QEMU's parallel preallocation is performed > only during VM start-up. We submitted this patch series mainly > because we observed that, even with the existing parallel mechanism, > launching large-size VMs still incurs prohibitive delays. (Bringing up > a 2 TB VM still requires more than 40 seconds for zeroing) > >> If you know that you will run such a VM (or something else) later, you >> could pre-zero the memory from user space by using a hugetlb-backed file >> and supplying that to QEMU as memory backend for the VM. Then, you can >> start your VM without any pre-zeroing. >> >> I guess that approach should work universally. Of course, there are >> limitations, as you would have to know how much memory an app needs, and >> have a way to supply that memory in form of a file to that app. > > Regarding user-space pre-zeroing, I agree that it is feasible once the > VM's memory footprint is known. We evaluated this approach internally; > however, in production environments, it is almost impossible to predict > the exact amount of memory a VM will require. Of course, you could preallocate to the expected maximum and then truncate the file to the size you need :) -- Cheers David